Fuel cell based, electrical generator apparatus utilizing solid oxide electrolyte fuel cells ("SOFC") arranged within a housing and surrounded by insulation are well known, and taught, for example, by U.S. Pat. Nos: 4,395,468 (Isenberg) and "Solid Oxide Fuel Cell", Westinghouse Electric Corporation, October, 1992, for tubular SOFC; 4,476,196 (Poppel, et al.) for flat plate SOFC; and 4,476,198 (Ackerman, et al.) for "corrugated" SOFC. The tubular type fuel cells can comprise an open or closed ended, axially elongated, ceramic tube air electrode material, which may be deposited on a ceramic support tube, completely covered by thin film ceramic, solid electrolyte material. The electrolyte layer is covered by cermet fuel electrode material, except for a thin, axially elongated, interconnection material. The flat plate type fuel cells can comprise a flat array of electrolyte and interconnect walls, where electrolyte walls contain thin, flat layers of cathode and anode materials sandwiching an electrolyte. The "corrugated" plate type fuel cells can comprise a triangular or corrugated honeycomb array of active anode, cathode, electrolyte and interconnect materials. Other fuel cells not having a solid electrolyte, such molten carbonate fuel cells are also well known, and can be utilized in the article and method of this invention.
Development studies of SOFC power plant systems have indicated the desirability of pressurized operation. This would permit operation with a coal gasifier as the fuel supply and/or use of a gas turbine generator as a bottoming cycle. Integration is thought commercially possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow.
Conventional combustor in a gas turbine system typically exhibit high nitrogen oxides (NOx) emissions, combustion driven oscillations and instabilities, excessive noise and low efficiencies. A typical diffusion flame combustor will produce about 200 parts per million NOx (corrected to 15% 0.sub.2) when operated at base load on natural gas fuel in a typical combustion turbine. Although significant advances have been made to mitigate these problems, it has proved difficult to design a practical, ultra-low NOx, high-turn-down ratio combustor due to poor flame stability characteristics. The combination of all the above factors results in pressurized SOFC generator module design being suitable as a replacement of conventional gas turbine combustor and applicable to more efficient combined cycle power plants required to meet increasingly stringent emission targets.
A variety of fuel cell use in power plant systems are described in the literature. In U.S. Pat. No. 3,972,731 (Bloomfield et al.), a pressurized fuel cell power plant is described. There, air is compressed by compressor apparatus, such as a compressor and turbine which are operably connected, which is powered by waste energy produced by the power plant in the form of a hot pressurized gaseous medium, such as fuel cell exhaust gases. These exhaust gases are delivered into the turbine, which drives the compressor for compressing air delivered to the fuel cells. In U.S. Pat. No. 5,413,879 (Domeracki et al.) a SOFC is also integrated into a gas turbine system. There, pre-heated, compressed air is supplied to a SOFC along with fuel, to produce electric power and a hot gas, which gas is further heated by combustion of unreacted fuel and oxygen remaining in the hot gas. This higher temperature gas is directed to a topping combustor that is supplied with a second stream of fuel, to produce a still further heated gas that is then expanded in a turbine.
U.S. Pat. No. 4,622,275 (Noguchi et al.) also describes a fuel cell power plant, where reformed, reactive fuel is fed to an anode of the cell, an expansion turbine connected to a compressor feeds compressed gas into the cathode of the cell, which compressed gas is mixed with anode exhaust gas which had been combusted. A variety of fuel cell types, in various system configurations is described by B. R. Gilbert et al. in "Fuel Cells Make Their CPI Moves", in Chemical Engineering, August 1995, pp. 92-96. Specifically, a conceptual design of a 1 MW commercial unit shows two molten carbonate fuel cell stacks and two associated reformers enclosed within a horizontal cylindrical vessel. There is no teaching of purge gas use. The same concept is also reviewed by S. E. Keuhn in "Molten-Carbonate Fuel Cell Demonstrates its Commercial Readiness", Power Engineering, March, 1995, p. 16.
Fuel cell pressurization, while advantageous in system performance, presents several practical difficulties to the SOFC generator designer, two of which are: (1) The pressure boundary must be able to withstand pressures up to 20 atmospheres. The pressure boundary of existing generators operating at one atmosphere pressure is the outside SOFC generator wall, which typically operates at temperatures between 600.degree. C. and 800.degree. C. Construction of a pressure boundary to operate at 20 atmospheres and 800.degree. C. would be difficult, even with exotic materials, and probably prohibitively expensive. Therefore, a pressure boundary with reduced wall temperature is desirable; (2) Because fuel and air are brought together within the SOFC generator, care must be taken to avoid-the potential of an unstable condition during startup and operation. This is only a limited concern at one atmosphere, since the explosive overpressure with the gaseous fuels used in the SOFC is about seven or eight times the absolute operating pressure. For atmospheric operation, the expected explosive overpressure would be about 115 psi (8.10kg/cm.sup.2) which existing designs can accommodate by mechanical strength alone. However, the expected explosive overpressure at 20 atmospheres is about 2315 psi (163kg/cm.sup.2). An enclosure designed to contain the explosion overpressure at 20 atmospheres would be extremely expensive. A protective containment system to prevent the accumulation of an explosive gas mixture is required. It would also be highly desirable to have a transportable, fuel cell based electrical generator apparatus in the 1MW (megawatt) to 10 MW class which can be assembled at the factory and shipped long distance to the power plant location. It is one of the objects of this invention to provide safe, transportable fuel cell modules and a method of transporting and operating such modules.
Accordingly, the invention resides in a fuel cell generator apparatus characterized by containing at least one fuel cell assembly module containing a plurality of fuel cells, each fuel cell having electrolyte between an oxidant electrode and a fuel electrode; where the module is enclosed by a module housing capable of withstanding temperatures over 600.degree. C.; where the module housing is surrounded by an axially-elongated pressure vessel having two ends, such that there is a purge gas space between the module housing and the pressure vessel; where the pressure vessel has a fuel gas inlet connecting to a module fuel gas inlet, an oxidant gas inlet connecting to a module oxidant gas inlet, an exhaust gas outlet connecting to a module exhaust gas outlet, and a purge gas inlet connecting to the purge gas space between the module housing and the pressure vessel; and where the purge gas space is effective to control any unreacted fuel gas flow from the module by dilution with purge gas. The fuel cells will generally operate at temperatures over about 650.degree. C., usually over about 650.degree. C. and up to about 1100.degree. C. The module housing and the fuel cells can operate in the "pressurized" mode, that is over about 2 atmospheres, or about 28.5 psi (pounds per square inch -- 2.0 kg/sq.cm), preferably at about 10 atmospheres. Gaseous oxidant channels from the oxidant inlet can connect to cooling ducts in the module housing walls to allow gaseous oxidant passage through the cooling ducts to the fuel cells, the gaseous oxidant acting as a cooling gas, and the fuel gas inlet can connect to the module fuel gas inlet through a common manifold.
The invention also resides in a method of operating a fuel cell generator apparatus characterized by the steps of: (1) passing oxidant gas and fuel gas, both being pressurized, through inlets and into a plurality of fuel cell assembly modules, each module containing a plurality of fuel cells, each fuel cell having electrolyte between an oxidant electrode and a fuel electrode, where the modules are each enclosed by a module housing capable of withstanding temperatures over 600.degree. C.; where the module housings are surrounded by an axially-elongated pressure vessel having two ends, such that there is a purge gas space between the module housings and the pressure vessel, the oxidant gas and fuel gas also passing through the pressure vessel enclosing the modules; (2) passing pressurized purge gas through the pressure vessel to circulate within the purge gas space, where the purge gas dilutes any unreacted fuel gas flow from the module; and (3) passing exhaust gas and circulated purge gas and any unreacted fuel gas out of the pressure vessel.
The generator apparatus can operate at interior temperatures up to about 1100.degree. C. in a flow of fuel, and oxidant such as oxygen or air. Thermocouples measure module housing temperature and if it is below about 520.degree. C. (fuel autoignition temperature) they effect shut off of the fuel inlet. The generator apparatus will also have associated with it and will be working in cooperation with well known auxiliaries, such as controls; an oxygen or air preheater; a fuel gas compressor; a fuel desulfurizer; an oxygen or air compressor which may be operably connected to a power turbine coupled to an electric generator; a purge gas compressor, which may be the same as the air compressor; a source of fuel gas and purge gas; heat exchangers; a heat recovery unit to recover heat from the hot fuel cell exhaust gases; and a topping combustor, to provide an electrical power generation system. This type power system could be, for example, part of an integrated, coal gasification/fuel cell-steam turbine combination power plant, featuring a plurality of coal gasifiers and fuel cell generator arrays or power blocks with associated DC/AC conversion switchgear. This type power system could also be part of a natural gas fired combustion turbine system, or the like.
The invention further resides in a method of transporting fuel cells in a fuel cell generator apparatus characterized by the steps of: (1) electronically connecting together a plurality of fuel cell assembly modules in a horizontal plane each module containing a plurality of fuel cells, (2) inserting the connected modules into an axially-elongated, horizontally disposed pressure vessel having two ends, to provide a low center of gravity generator apparatus, where there is a purge gas space between the modules and the pressure vessel; (3) providing inlets into the pressure vessel for feed fuel, feel oxidant, and purge gas, and outlets for exhaust gases; and (4) transporting the generator apparatus with its pressure vessel and contained fuel cell assembly modules.
This pressurized fuel cell generator apparatus design provides a unique safety feature for fuel cell operation, an easily transportable assembly, and eliminates nitrogen oxide emissions. By combining several modules within the pressure vessel, it is possible to operate the fuel cells at high pressure, typically at 10 atmospheres, thus greatly improving overall SOFC module voltage, efficiency and power output to the extent that it becomes feasible to integrate this apparatus with an industrial gas turbine in a high efficiency combined cycle power plant. This integration is possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow. In other words, the SOFC module acts as a conventional combustor in a gas turbine and it provides the volumetric flow rate, at the required temperature and pressure, which is discharged through the turbine.
Integration of these pressurized SOFC modules with conventional gas turbines in a combined cycle power plant, will boost overall electrical efficiencies to 65-70%, values presently unmatched by any other power generation technology. The pressurized SOFC generator modules will result in a design that can be used with the full range of existing commercial combustion turbines and will not require modification to the units other than in the fuel and combustor system. Because of its modular design, this pressurized SOFC generator concept is scalable and it may be applied to large size SOFC Combustion Turbine systems without much engineering effort. Other additional advantages include, but are not limited to, lower pressure on the air feed side, the capability of modulated heat output and smaller air-to-exhaust recuperators and ducting.